A Study of Hairlining

A test for the hairlining tendencies of enamels is presented. A curved 4- by 8-in. piece of 18-gauge enameling iron is used as a test piece in which a simulated service condition of strain is produced by a 2- by 2-in. piece of 16-gauge enameling iron placed on the top center of the test piece. Before enamel application the specimen is curved lengthwise in a mold of 1 1/2-ft. radius of curvature with a resulting permanent distortion of approximately 1/8 in.; the strain condition is obtained by an unattached straining piece. These two conditions enable good reproducibility of the direction and magnitude of the distortion due to uneven heating during firing, and the resulting pattern of hairlines varies only as influenced by enameling variables. The stepwise process of hairline formation is described with the aid of photographs and photomicrographs of enamel surfaces and cross sections. The effects of ground-coat and cover-coat variables and the significance of proper firing schedules are presented and discussed.